Read the German Wikipedia article on the subject which shows German signs in short tons (non-metric unit): https://de.wikipedia.org/wiki/Militärische_Lastenklasse
What I don't understand is your claim that "you could just as well consider them metric tons". These are short tons with caveats, i.e. the eventual number depends on more than just the raw weight, but the raw weight is one aspect of the calculation.
So if you were to make that calculation on the basis of metric instead of lbs how aren't you going to introduce something like a 10% error in the MLC you come up with?